1. Field of the Invention
This invention relates to an electrical circuit controller for protecting a power circuit and a load powered by the power circuit, and more particularly to a contactor having an external current limiting conductive polymer resistance for providing current limiting protection in the power circuit.
2. Background of Information
Electromagnetic contactors are electrically operated switches used for controlling motors and other types of electrical loads. Contactors include a set of movable electrical contacts which are brought into contact with a set of fixed electrical contacts to close the contactor and connect a power line to the load. The set of movable contacts are separated from the set of fixed contacts to open the contactor.
Contactors also include a magnetic circuit having a fixed magnet and a movable armature with an air gap therebetween when the contactor is opened. An electromagnetic coil is controllable upon command to interact with a source of voltage for electromagnetically accelerating the armature towards the fixed magnet, thus reducing the air gap. Disposed on the armature is the set of movable contacts. The complementary set of fixed contacts are fixedly disposed within the contactor case and engage the movable contacts as the magnetic circuit is energized and the armature is moved. The load and voltage source therefor are interconnected with the fixed contacts and become interconnected with each other as the movable contacts make with the fixed contacts.
Contact erosion in contactors primarily occurs during a contact breaking cycle. During such cycle, the separable contacts (i.e., the fixed contacts and the movable contacts) part and the current flowing therethrough forms an arc. Continued arcing eventually interferes with the ability of the separable contacts to conduct electricity. The surface of the separable contacts may become eroded, pitted or may have carbon build-up.
A motor-starting contactor with a thermal overload protection relay system is called a motor starter. The purpose of an overload relay is to sense heat produced by line current and "trip" or stop the motor if the retained heat exceeds an acceptable level. State of the art overload protection relay systems include current sensors which output a voltage proportional to the current. After an analog-to-digital conversion of the voltage, a microprocessor squares and integrates the converted digital value to achieve a true measure of motor heating. This approach provides an accurate degree of motor protection. However, the overload relay cannot effectively interrupt the entire short circuit current. Without additional circuit protection, the short circuit current may damage the separable contacts of the contactor.
Circuit breakers are generally used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload fault or a relatively high level short circuit condition. An overload fault condition is normally about 125-600 percent of the nominal current rating of the circuit breaker. A high level short circuit condition can be 1000 percent or more of the nominal current rating of the circuit breaker. For example, in a bolted three phase line-ground fault, the short circuit current may exceed 100 KA at 480 VAC.
In conventional motor starter installations, a three-phase power source powers a motor through a circuit breaker and a separate motor starter having an overload protection relay. The circuit breaker generally provides both overcurrent protection and power circuit disconnection functions. Under short circuit fault conditions, the circuit breaker acts first to protect the power circuit. This is because the motor starter trip characteristics are generally designed for interrupting, after an I.sup.2 t time delay period, persistent overload currents associated with motor overloads. In the event of a motor starter failure, involving welded contacts, the circuit breaker may also be used to disconnect the power source from the motor.
Positive temperature coefficient (PTC) resistive elements are known to be used as reusable fuses. A PTC resistive element exhibits a relatively low resistance to a flow of electrical current when the current is below a threshold value. Current above the threshold value flowing through the PTC resistive element causes resistive heating of the PTC element. A rise in internal temperature of the PTC resistive element to above a transition temperature causes the PTC element to transition into a state of high resistance, thereby limiting current flow through the PTC element and the circuit containing it.
As stated in PTC Application Notes, Keystone Carbon Company Bulletin T-929, page 37, "[t]he dramatic rise in resistance of a PTC at the transition temperature makes it an ideal candidate for current limiting applications. For currents below the limiting current (I.sub.L), the power being generated in the unit is insufficient to heat the PTC to its transition temperature. However, when abnormally high-fault currents flow, the resistance of the PTC increases at such a rapid rate that any further increase in power dissipation results in a reduction in current."
Examples of PTC resistive devices include silicon carbide, tungsten, polycrystalline ceramic barium titanate, barium and strontium titanate, and current limiting conductive polymers.
Current limiting conductive polymers are known in the art to be useful for limiting electrical current. For example, Raychem Corporation manufactures and markets a current limiting polymer under the trademark PolySwitch.TM.. Current limiting polymers having PTC characteristics are disclosed in U.S. Pat. Nos. 4,545,926; 4,560,498 and 4,775,778, all owned by Raychem Corporation. Current limiting polymers typically comprise cross-linked polyethylene, heavily doped with carbon. PTC's typically have a low electrical resistance when conducting current below a threshold value (i.e., when the PTC is relatively cool). When current flowing through the PTC exceeds the threshold value, resistive heating produces a rise in the internal temperature of the PTC, causing a reduction in conductivity (i.e., an increase in electrical resistance). The power dissipated in the PTC is proportional to the resistance multiplied by the square of the current. Therefore, for a constant or increasing current flow, an increase in resistance leads to a further increase of resistance. This increase in resistance is, thus, quite rapid. Typically, the increase in resistance is virtually a step function once the magnitude of the current (and the resulting internal temperature of the polymer) surpasses the threshold value.
The change in resistance of the PTC upon passing the threshold value is quite large. For example, the resistance of a current limiting polymer upon passing the threshold value may rapidly increase by a factor of 1,000 to greater than 4,000 times its initial resistance. Whenever the PTC is connected in a power system in series with a load, the increase in resistance of the PTC increases the total load resistance and, thus, substantially reduces the load current. However, the increase in resistance of the PTC produces a corresponding increase in the voltage drop across the PTC and a corresponding decrease in the voltage drop across the load. In this manner, a larger portion of the power from the power line is dissipated in the PTC as heat, as opposed to being dissipated by the load. Depending on the application (i.e., the line voltage and the load resistance), the voltage drop across a PTC which has transitioned to a high resistance state may be substantial and may result in destruction of the PTC. This is especially true when a conductive polymer is used as the PTC. Furthermore, PTC's are known to exhibit negative temperature coefficient (NTC) resistance characteristics if the internal temperature of the PTC goes much beyond the PTC threshold value. If heated to the NTC threshold value, the resistance of the PTC decreases.
Placing a PTC in thermal communication with a heat producing component is known in the art. Such an arrangement is disclosed in U.S. Pat. Nos. 4,780,598 and 5,064,997, both owned by Raythem Corporation. As disclosed in U.S. Pat. Nos. 4,780,598 and 5,064,997, the heat producing component is a voltage-dependent resistor. The voltage-dependent resistor and the PTC are electrically coupled in a series circuit with components to be protected from excessive current flow. The heat producing component radiates heat to the PTC to accelerate its transition into a state of high resistance to protect the other circuit components.
In contrast to the use of circuit breakers which interrupt short circuit fault currents, various proposals have been advanced for limiting such fault currents in conventional motor starter installations. International Application Number PCT/SE91/00076 discloses a circuit breaker comprising a trip circuit, a trip coil, a set of contacts responsive to the trip coil, and a positive temperature coefficient conductive polymer thermistor in series with the set of contacts. The thermistor limits the magnitude of the short-circuit current. However, the coil of the trip circuit rapidly opens the contacts within approximately 5 ms after a fault current exceeds five to ten times rated current.
It is well known that conductive polymers have a relatively limited heat capacity characteristic. Thus, prior art proposals utilize a separate overcurrent protective circuit, such as a circuit breaker trip coil or a motor starter overload relay, to open the power circuit within two power line cycles. In this manner, the conductive polymer current limiter is protected from excessive temperatures during short circuit fault conditions. Therefore, the prior art proposals which use conductive polymers either require a circuit breaker for rapidly interrupting the short circuit fault current or, else, require a contactor having an overload relay or a special sensor for interrupting the current flow after a limited number of power line cycles. In either case, the circuit breaker, the overload relay or the sensor must act relatively quickly (i.e., within a few power line cycles) after a short circuit fault in order to protect the conductive polymer current limiter.
Prior art applications, of necessity, rapidly interrupted the short circuit fault current before the temperature of the conductive polymer rose to a destructive level. In the same manner, such prior applications rapidly interrupted the short circuit fault current before the resistance of the conductive polymer rose to a sufficient resistance which divided substantially all of the power line voltage across the conductive polymer. Furthermore, such prior applications, of necessity, rapidly interrupted the short circuit fault current flowing through the power circuit before the current and load side voltage were substantially reduced. Accordingly, contact arcing and contact erosion of the separable contacts of the contactor persist.
There is a need, therefore, for a contactor which minimizes contact arcing and contact erosion under short circuit conditions.
There is a more particular need for such a contactor which protects a power circuit from damage under short circuit conditions.
There is an even more particular need for such a contactor which is not damaged or seriously degraded under short circuit conditions.
There is another even more particular need for a circuit controller which protects a power circuit from damage without requiring a circuit breaker.